Encrypting of communications using a transmitting/receiving apparatus via key information based on a multi-level code signal and a pseudo-random number sequence for modulation with an information signal

ABSTRACT

A highly concealable data communication apparatus based on an astronomical complexity and causing an eavesdropper to take a significantly increased time to analyze a cipher text, is provided. In a multi-level code generation section  111   a , a random number sequence generation section  141  generates, based on predetermined key information  11,  a plurality of modulation pseudo-random number sequences. The plurality of modulation pseudo-random number sequences is inputted to a multi-level conversion section  142  as a part of an input bit sequence which is converted into a multi-level code sequence  12.  A multi-level processing section  111   b  combines the multi-level code sequence  12  and information data  10,  and generates a multi-level signal  13  having a plurality of levels corresponding to a combination of the multi-level code sequence  12  and the information data  10.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The preset invention relates to an apparatus and a method for performingsecret communication in order to avoid illegal eavesdropping andinterception by a third party and, more particularly, relates to a datatransmitting apparatus, a data receiving apparatus and a datatransmitting method for performing data communication through selectingand setting a specific encoding/decoding (modulating/demodulating)method between a legitimate transmitter and a legitimate receiver.

2. Description of the Related Art

Conventionally, in order to perform secret communication betweenspecific parties, there has been adopted a structure for realizingsecret communication by sharing key information for encoding/decodingbetween transmitting and receiving ends and by performing, based on thekey information, an operation/inverse operation on information data(plain text) to be transmitted, in a mathematical manner. FIG. 17 is ablock diagram showing a structure of a conventional data communicationapparatus based on the above-described structure.

In FIG. 17, the conventional data communication apparatus has aconfiguration in which a data transmitting apparatus 9001 and a datareceiving apparatus 9002 are connected to each other via a transmissionline 913. The data transmitting apparatus 9001 includes an encodingsection 911 and a modulator section 912. The data receiving apparatus9002 includes a demodulator section 914 and a decoding section 915.

In the data transmitting apparatus 9001, information data 90 and firstkey information 91 are inputted to the encoding section 911. Theencoding section 911 encodes (modulates), based on the first keyinformation 91, the information data 90. The modulator section 912converts, in a predetermined demodulation method, the information data90 encoded by the encoding section 911 into a modulated (modulating)signal 94 which is then transmitted to the transmission line 913.

In the data receiving apparatus 9002, the demodulator section 914demodulates, in a predetermined demodulation method, the modulated(modulating) signal 94 transmitted via the transmission line 913. To thedecoding section 915, second key information 96 which has the samecontent as the first key information 91 is inputted. The decodingsection 915 demodulates (decrypts), based on the second key information96, the modulated (modulating) signal 94 and then outputs informationdata 98.

Here, eavesdropping by a third party will be described by using aneavesdropper receiving apparatus 9003. In FIG. 17, eavesdropperreceiving apparatus 9003 includes an eavesdropper demodulator section916 and an eavesdropper decoding section 917.

The eavesdropper demodulator section 916 demodulates, in a predetermineddemodulation method, the modulated (modulating) signal 94 transmittedvia the transmission line 913. The eavesdropper decoding section 917attempts, based on third key information 99, decoding of a signaldemodulated by the eavesdropper demodulator section 916. Here, since theeavesdropper decoding section 917 attempts, based on the third keyinformation 99 which is different in content from the first keyinformation 91, decoding of the signal demodulated by the eavesdropperdemodulator section 916, the information data 98 cannot be reproducedaccurately.

A mathematical encryption (or also referred to as a computationalencryption or a software encryption) technique based on suchmathematical operation may be applicable to an access system describedin Japanese Laid-Open Patent Publication No. 9-205420 (hereinafterreferred to as Patent Document 1), for example. That is, in a PON(Passive Optical Network) system in which an optical signal transmittedfrom an optical transmitter is divided by an optical coupler anddistributed to optical receivers at a plurality of optical subscribers'houses, such optical signals that are not desired and aimed at anothersubscribers are inputted to each of the optical receivers. Therefore,the PON system encrypts information data for each of the subscribers byusing key information which is different by the subscribers, therebypreventing a leakage/eavesdropping of mutual information data andrealizing safe data communication.

Further, the mathematical encryption technique is described in“Cryptography and Network Security: Principles and Practice” translatedby Keiichiro Ishibashi et al., Pearson Education, 2001 (hereinafterreferred to as Non-patent Document 1) and “Applied Cryptography”translated by Mayumi Adachi et al., Softbank publishing, 2003(hereinafter referred to as Non-patent Document 2).

Among the mathematical encryption, a method called a stream encryptionhas a simple structure in which a cipher text is generated by performingan XOR operation between a pseudo-random number sequence outputted by apseudo-random number generator and information data (a plain text) to beencrypted, and thus is advantageous for an increase in speed. On theother hand, the method is disadvantageous in that security in the streamencryption depends only on the pseudo-random number generator. That is,if the eavesdropper can obtain a combination of the plain text and thecipher text, the pseudo-random number series can be identifiedaccurately (this is generally called a known-plain-text attack).Further, since an initial value of the pseudo-random number generator,that is, the key information and the pseudo-random number seriescorrespond to each other uniquely, the key information can be figuredout certainly if any decryption algorithm is applied. Further, aprocessing speed of a computer has been improved remarkably in recentyears, and thus there has been a problem in that there is an increasingdanger of decryption of the cipher text within a practical time period.

BRIEF SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a highlyconcealable data communication apparatus which causes the eavesdropperto take a significantly increased effort and time to analyze the ciphertext, compared to a conventional stream encryption, by introducing anuncertain element into a relation among key information, a pseudo-randomnumber sequence and a cipher text.

The present invention is directed to a data transmitting apparatus forencrypting information data by using predetermined key information andperforming secret communication with a receiving apparatus. To attainthe object mentioned above, the data receiving apparatus of the presentinvention includes: a multi-level code generation section forgenerating, based on the predetermined key information, a multi-levelcode sequence in which a signal level changes so as to approximatelyrepresent random numbers; a multi-level processing section for combiningthe multi-level code sequence and the information data and generating amulti-level signal having a plurality of levels corresponding to acombination of the multi-level code sequence and the information data;and a modulator section for treating the multi-level signal withpredetermined modulation processing and outputting a modulated signal.Further, the multi-level code generation section includes: a randomnumber sequence generation section for generating, based on thepredetermined key information, a plurality of modulation pseudo-randomnumber sequences; and a multi-level conversion section to which aplurality of bit sequences including at least a part of the plurality ofmodulation pseudo-random number sequences is inputted as an input bitsequence and which converts the input bit sequence into the multi-levelcode sequence. The input bit sequence to the multi-level conversionsection is greater in a number of digits than each of the plurality ofmodulation pseudo-random number sequences generated by the random numbersequence generation section.

Preferably, the multi-level processing section allocates differentvalues of the information data to adjoining multi-levels of themulti-level signal.

At least one of the plurality of modulation pseudo-random numbersequences is inputted to the multi-level conversion section as alowest-order bit of the input bit sequence.

Preferably, the multi-level code generation section further includes aphysical random number generation section for generating one or morephysical random number sequences. In this case, the one or more physicalrandom number sequences are inputted, to the multi-level conversionsection, as remaining bit sequences of the input bit sequence afterexcluding the at least a part of the plurality of the modulationpseudo-random number sequences.

Further, fixed values maybe inputted, to the multi-level conversionsection, as remaining bit sequences of the input bit sequence afterexcluding the at least a part of the plurality of the modulationpseudo-random number sequences.

Preferably, the multi-level code generation section further includes aphysical random number generation section for generating one or morephysical random number sequences. In this case, the one or more physicalrandom number sequences are inputted to the multi-level conversionsection as a part of the plurality of the bit sequences of the input bitsequence after excluding the at least a part of the plurality of themodulation pseudo-random number sequences, and fixed values areinputted, as remaining bit sequences thereof.

Further, a signal generated based on a predetermined rule may beinputted, to the multi-level conversion section, as remaining bitsequences of the input bit sequence excluding the at least a part of theplurality of the modulation pseudo-random number sequences. The signalgenerated based on the predetermined rule may be generated by delaying apart or a whole of the plurality of modulation pseudo-random numbersequences by a predetermined time period.

A condition needs to be satisfied where a ratio of an informationamplitude, which corresponds to an amplitude of the information data, toa fluctuation width of the multi-level signal is greater than asignal-to-noise ratio acceptable to a legitimate receiving party.

Preferably, the random number sequence generation section includes: apseudo-random number generation section for generating, based on thepredetermined key information, a pseudo-random number series which is ina binary format; and a serial/parallel conversion section for performingserial/parallel conversion of the pseudo-random number series generatedby the pseudo-random number generation section, and outputting theplurality of modulation pseudo-random number sequences.

Further, the random number sequence generation section may includes: apseudo-random number generation section for generating, based on thepredetermined key information, a pseudo-random number series which is ina binary format; a plurality of serial/parallel conversion sections forperforming serial/parallel conversion of the pseudo-random number seriesgenerated by the pseudo-random number generation section and outputtingthe plurality of modulation pseudo-random number sequences; a firstswitch for switching, based on a rate selection signal, an outputdestination of the pseudo-random number series generated by thepseudo-random number generation section, between the plurality ofserial/parallel conversion sections; and a second switch for selecting,based on the rate selection signal, and outputting the plurality ofmodulation pseudo-random number sequences outputted from the pluralityof serial/parallel conversion sections. The plurality of serial/parallelconversion sections output respectively different numbers of theplurality of modulation pseudo-random number sequences.

Further, the present invention is directed to a data receiving apparatusfor receiving information data encrypted by using predetermined keyinformation and performing secret communication with a transmittingapparatus. To attain the object mentioned above, the data receivingapparatus includes: a multi-level code generation section forgenerating, based on the predetermined key information, a multi-levelcode sequence in which a signal level changes so as to approximatelyrepresent random numbers; a demodulator section for demodulating, in apredetermined demodulation method, a modulated signal received from thetransmitting apparatus so as to be outputted as a multi-level signalhaving a plurality of levels corresponding to a combination of theinformation data and the multi-level code sequence; and an decisionsection for deciding, based on the multi-level code sequence, theinformation data from the multi-level signal. The multi-level codegeneration section includes: a random number sequence generation sectionfor generating, based on the predetermined key information, a pluralityof demodulation pseudo-random number sequences; and a multi-levelconversion section to which a plurality of bit sequences including atleast a part of the plurality of demodulation pseudo-random numbersequences are inputted as an input bit sequence, and which converts theinput bit sequence into the multi-level code sequence. The input bitsequence to the multi-level conversion section is greater in a number ofdigits than each of the plurality of demodulation pseudo-random numbersequences generated by the random number sequence generation section.

Fixed values are inputted, to the multi-level conversion section, asremaining bit sequences of the input bit sequence excluding the at leasta part of the plurality of demodulation pseudo-random number sequences.

A signal generated based on a predetermined rule may be inputted, to themulti-level conversion section, as remaining bit sequences of the inputbit sequence excluding the at least a part of the plurality ofdemodulation pseudo-random number sequences. The signal generated basedon the predetermined rule may be generated by delaying a part or a wholeof the plurality of demodulation pseudo-random number sequences by apredetermined time period.

A condition needs to be satisfied where a ratio of an informationamplitude corresponding to an amplitude of the information data to afluctuation width of the multi-level signal corresponding to remainingbit sequences of the input bit sequence to the multi-level conversionsection, after excluding the plurality of demodulation pseudo-randomnumber sequences, is greater than a signal-to-noise ratio acceptable toa legitimate receiving party.

Preferably, the random number sequence generation section includes: apseudo-random number generation section for generating, based on thepredetermined key information, a pseudo-random number series which is ina binary format; and a serial/parallel conversion section for performingserial/parallel conversion of the pseudo-random number series generatedby the pseudo-random number generation section, and outputting theplurality of demodulation pseudo-random number sequences.

Further, the random number sequence generation section may include: apseudo-random number generation section for generating, based on thepredetermined key information, a pseudo-random number series which is ina binary format; a plurality of serial/parallel conversion sections forperforming serial/parallel conversion of the pseudo-random number seriesgenerated by the pseudo-random number generation section and outputtingthe plurality of demodulation pseudo-random number sequences; a firstswitch for switching, based on a rate selection signal, an outputdestination of the pseudo-random number series generated by thepseudo-random number generation section, between the plurality of theserial/parallel conversion sections; and a second switch for selecting,based on the rate selection signal, and outputting the plurality ofdemodulation pseudo-random number sequences outputted from the pluralityof serial/parallel conversion sections. The plurality of serial/parallelconversion sections outputs respectively different numbers of theplurality of demodulation pseudo-random number sequences.

Further, the data transmission apparatus mentioned above and processingprocedures performed by the modulation section maybe regarded as a datatransmission method for causing a series of processing procedures to beexecuted. That is, the data transmission method includes: a multi-levelcode generation step of generating, based on the predetermined keyinformation, a multi-level code sequence in which a signal level changesso as to approximately represent random numbers; a step of combining themulti-level code sequence and the information data and generating amulti-level signal having a plurality of levels corresponding to acombination of the multi-level code sequence and the information data;and a modulation step of treating the multi-level signal withpredetermined modulation processing and outputting a modulated signal.The multi-level code generation step includes: a random number sequencegeneration step of generating, based on the predetermined keyinformation, a plurality of modulation pseudo-random number sequences;and a multi-level conversion step in which a plurality of bit sequencesincluding at least apart of the plurality of modulation pseudo-randomnumber sequences is inputted as an input bit sequence and the input bitsequences are converted into the multi-level code sequence. The inputbit sequence is greater in a number of digits than each of the pluralityof modulation pseudo-random number sequences.

Further, respective processing procedures performed by the multi-levelcode generation section, the demodulation section, and the decisionsection which are included in the data receiving apparatus mentionedabove maybe regarded as a data receiving method for causing a series ofprocessing procedures to be executed. That is, the data receiving methodincludes: a multi-level code generation step of generating, based on thepredetermined key information, a multi-level code sequence in which asignal level changes so as to approximately represent random numbers; ademodulation step of demodulating, in a predetermined demodulationmethod, a modulated signal received from the transmitting apparatus soas to be outputted as a multi-level signal having a plurality of levelscorresponding to a combination of the information data and themulti-level code sequence; and an decision step of deciding, based onthe multi-level code sequence, the information data from the multi-levelsignal. The multi-level code generation step includes: a random numbersequence generation step of generating, based on the predetermined keyinformation, a plurality of demodulation pseudo-random number sequences;and a multi-level conversion step in which a plurality of bit sequencesincluding at least a part of the plurality of demodulation pseudo-randomnumber sequences are inputted as an input bit sequence, and the inputbit sequence is converted into the multi-level code sequence. The inputbit sequence is greater in a number of digits than each of the pluralityof demodulation pseudo-random number sequences.

The data communication apparatus of the present inventionencodes/modulates, based on key information, information data into amulti-level signal which is then to be transmitted, decodes/demodulates,based on the key information, a received multi-level signal andoptimizes a signal-to-noise power ratio of the multi-level signal,thereby causing a cipher text obtained by an eavesdropper to beerroneous. As a result, the eavesdropper needs to perform decodingconsidering that a correct cipher text is highly likely to be differentfrom what the eavesdropper has obtained, and thus the number of attemptsrequired for the decoding, that is the amount of computing, will beincreased compared to a case of no error. Accordingly, security againsteavesdropping can be improved. Further, the intervals between the levelsof the multi-level signal are set appropriately, whereby an increase ina rate of the cipher text pseudo-random number generator used within theapparatus can be kept at the lowest level.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an example of a configuration of adata communication apparatus according to the present invention;

FIG. 2 is a diagram illustrating a waveform of a transmission signal ofthe data communication apparatus according to the first embodiment ofthe present invention;

FIG. 3. is a diagram illustrating names of the waveform of thetransmission signal of the data communication apparatus according to thefirst embodiment of the present invention;

FIG. 4 is a diagram illustrating quality of the transmission signal ofthe data communication apparatus according to the first embodiment ofthe present invention;

FIG. 5 is a diagram illustrating the quality of another transmissionsignal of the data communication apparatus according to the firstembodiment of the present invention;

FIG. 6 is a block diagram showing an example of a detailed configurationof a first multi-level code generation section 111 a according to asecond embodiment of the present invention;

FIG. 7 is a block diagram showing an example of a detailed configurationof a second multi-level code generation section 212 a according to thesecond embodiment of the present invention;

FIG. 8 is a diagram illustrating a signal format used for a datatransmitting apparatus according to the second embodiment of the presentinvention;

FIG. 9 is a block diagram showing an example of a detailed configurationof a first multi-level code generation section 111 a according to athird embodiment of the present invention;

FIG. 10 is a block diagram showing an example of a detailedconfiguration of a second multi-level code generation section 212 aaccording to the third embodiment of the present invention;

FIG. 11 is a diagram illustrating a signal format used for a datatransmitting apparatus according to the third embodiment of the presentinvention;

FIG. 12 is a block diagram showing an example of a detailedconfiguration of a first multi-level code generation section 111 aaccording to a fourth embodiment of the present invention;

FIG. 13 is a diagram illustrating a signal format used for a datatransmitting apparatus according to the fourth embodiment of the presentinvention;

FIG. 14A is a block diagram showing an example of another configurationof the first multi-level code generation section 111 a according to thefourth embodiment of the present invention;

FIG. 14B is a block diagram showing an example of another configurationof the first multi-level code generation section 111 a according to thefourth embodiment of the present invention;

FIG. 15 is a diagram illustrating another signal format used for thedata transmitting apparatus according to the fourth embodiment of thepresent invention;

FIG. 16 is a block diagram showing an example of a detailedconfiguration of a first random number sequence generation section 141according to a fifth embodiment of the present invention; and

FIG. 17 is a block diagram showing a configuration of a conventionaldata communication apparatus.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiment of the present invention will be described, withreference to drawings.

First Embodiment

FIG. 1 is a block diagram showing an example of a configuration of adata communication apparatus according to the present invention. In FIG.1, the data communication apparatus according to the first embodimenthas a configuration in which a data transmitting apparatus 1101 and adata receiving apparatus 1201 are connected to each other via atransmission line 110. The data transmitting apparatus 1101 includes amulti-level encoding section 111 and a modulator section 112. Themulti-level encoding section 111 includes a first multi-level codegeneration section 111 a and a multi-level processing section 111 b. Thedata receiving apparatus 1201 includes a demodulator section 211 and amulti-level decoding section 212. The multi-level decoding section 212includes a second multi-level code generation section 212 a and adecision section 212 b. A metal line such as a LAN cable or a coaxialline, or an optical waveguide such as an optical-fiber cable can be usedas the transmission line 110. Further the transmission line 110 is notlimited to a wired cable such as the LAN cable, but can be free spacewhich enables a wireless signal to be transmitted.

FIG. 2 is a diagram illustrating a waveform of a transmission signal ofthe data communication apparatus according to the first embodiment ofthe present invention. FIG. 3 is a diagram illustrating names of thewaveform of the transmission signal of the data communication apparatusaccording to the first embodiment of the present invention. FIG. 4 is adiagram illustrating quality of the transmission signal of the datacommunication apparatus according to the first embodiment of the presentinvention. Hereinafter, an action of the data communication apparatusaccording to the first embodiment of the present invention will bedescribed, with reference to FIGS. 1 to 4.

The first multi-level code generation section 111 a generates, based onpredetermined first key information 11, a multi-level code sequence 12((b) of FIG. 2) in which a signal level changes so as to approximatelyrepresent random numbers. The multi-level code sequence 12 ((b) of FIG.2) and information data 10 ((a) of FIG. 2) are inputted to themulti-level processing section 111 b. The multi-level processing section111 b combines the multi-level code sequence 12 and the information data10 in accordance with a predetermined procedure, and generates amulti-level signal 13 ((c) of FIG. 2) having a plurality of levelscorresponding to a combination of the multi-level code sequence 12 andthe information data 10. For example, in the case where a level of themulti-level code sequence 12 changes to c1/c5/c3/c4 with respect to timeslots t1/t2/t3/t4, the multi-level processing section 111 b regards themulti-level code sequence 12 as a bias level, adds the information data10 to the multi-level code sequence 12, and then generates themulti-level signal 13 in which a signal level changes to L1/L8/L6/L4.The modulator section 112 modulates the multi-level signal 13 in apredetermined modulation method, and outputs the modulated multi-levelsignal 13 as a modulated (modulating) signal 14 to the transmission line110.

Here, as shown in FIG. 3, an amplitude of the information data 10 isreferred to as an “information amplitude”, a total amplitude of themulti-level signal 13 is referred to as a “multi-level signalamplitude”, pairs of levels (L1, L4)/(L2, L5)/(L3, L6)/(L4, L7)/(L5, L8)which the multi-level signal 13 may obtain corresponding to the levelsc1/c2/c3/c4/c5 of the multi-level code sequence 12 are respectivelyreferred to as first to fifth “bases”, and a minimum interval betweensignal levels of the multi-level signal 13 is referred to as a “stepwidth”.

The demodulator section 211 demodulates the modulated signal 14transmitted via the transmission line 110, and reproduces a multi-levelsignal 15. The second multi-level code generation section 212 apreviously shares second key information 16 which has the same contentas the first key information 11, and based on the second key information16, generates a multi-level code sequence 17. The decision section 212 breceives the multi-level signal 15 and reproduces information data 18 bydeciding (binary determination) a value of the information data 18 usingthe multi-level code sequence 17 as a threshold. Here, the modulated(modulating) signal 14 which is modulated in a predetermined modulationmethod and is transmitted/received between the modulator section 112 andthe demodulator section 211 via the transmission line 110, is a signalobtained by modulating an electromagnetic wave (electromagnetic field)or a light wave using the multi-level signal 13.

Note that, the multi-level processing section 111 b may generate themulti-level signal 13 by using any method, in addition to a method ofgenerating the multi-level signal 13 by adding the information data 10and the multi-level code sequence 12 as above described. For example,the multi-level processing section 111 b may generate the multi-levelsignal 13 by modulating, based on the information data 10, an amplitudeof the levels of the multi-level code sequence 12. Alternatively, themulti-level processing section 111 b may generate the multi-level signal13 by reading out consecutively, from a memory having levels of themulti-level signal 13 previously stored therein, the levels of themulti-level signal 13, which are corresponding to the combination of theinformation data 10 and the multi-level code sequence 12.

Further, in FIG. 2 and FIG. 3, the levels of the multi-level signal 13are represented as 8 levels, but the levels of the multi-level signal 13are not limited to the representation. Further, the informationamplitude is represented as three times or integer times of the stepwidth of the multi-level signal 13, but the information amplitude is notlimited to the representation. The information amplitude may be anyinteger times of the step width of the multi-level signal 13, or is notnecessarily integer times thereof. Further, in FIG. 2 and FIG. 3, eachof the levels of the multi-level code sequence 12 is located so as to beat an approximate center between each of the levels of the multi-levelsignal 13, but each of the levels of the multi-level code sequence 12 isnot limited to such a location. For example, each of the levels of themulti-level code sequence 12 is not necessarily at the approximatecenter between each of the levels of the multi-level signal 13, or maycoincide with each of the levels of the multi-level signal 13. Further,the above description is based on an assumption that the multi-levelcode sequence 12 and the information data 10 are identical in a changerate to each other and also in a synchronous relation, but the changerate of either of the multi-level code sequence 12 or the informationdata 10 maybe faster (or slower) than the change rate of another, or themulti-level code sequence 12 and the information data 10 are in anasynchronous relation.

Next, an action of eavesdropping by a third party will be described. Itis assumed that the third party, who is an eavesdropper, decodes themodulated (modulating) signal 14 by using a configuration correspondingto the data receiving apparatus 1201 held by a legitimate receivingparty or a further sophisticated data receiving apparatus (hereinafterreferred to as an eavesdropper data receiving apparatus). Theeavesdropper data receiving apparatus reproduces the multi-level signal15 by demodulating the modulated (modulating) signal 14. However, theeavesdropper data receiving apparatus does not share the key informationwith the data transmitting apparatus 1101, and thus, unlike the datareceiving apparatus 1201, the eavesdropper data receiving apparatuscannot generate, based on the key information, the multi-level codesequence 17. Therefore, the eavesdropper data receiving apparatus cannotperform binary determination of the multi-level signal 15 by using themulti-level code sequence 17 as a reference.

As an action of the eavesdropping which maybe possible under thesecircumstances, there is a method of identifying all the levels of themulti-level signal 15 (generally referred to as “all-possible attacks”).That is, the eavesdropper data receiving apparatus performs adetermination of the multi-level signal 15 by preparing thresholdscorresponding to all possible intervals between the signal levels whichthe multi-level signal 15 may obtain, and attempts an extraction ofcorrect key information or information data by analyzing a result of thedetermination. For example, the eavesdropper data receiving apparatussets all the levels c0/c1/c2/c3/c4/c5/c6 of the multi-level codesequence 12 shown in FIG. 2 as the thresholds, performs the multi-leveldetermination of the multi-level signal 15, and then attempts theextraction of the correct key information or the information data.

However, in an actual transmission system, a noise occurs due to variousfactors, and the noise is overlapped on the modulated (modulating)signal 14, whereby the levels of the multi-level signal 15 fluctuatestemporally/instantaneously as shown in FIG. 4. In this case, an SN ratio(a signal-to-noise intensity ratio) of a signal to be determined (themulti-level signal 15) by the legitimate receiving party (the datareceiving apparatus 1201) is determined based on a ratio of theinformation amplitude to a noise level of the multi-level signal 15. Onthe other hand, the SN ratio of the signal to be determined (themulti-level signal 15) by the eavesdropper data receiving apparatus isdetermined based on a ratio of the step width to the noise level of themulti-level signal 15.

Therefore, in the case where a condition of the noise level contained inthe signal to be determined is fixed, the SN ratio of the signal to bedetermined by the eavesdropper data receiving apparatus is relativelysmaller than that by the data receiving apparatus 1201, and thus atransmission feature (an error rate) of the eavesdropper data receivingapparatus deteriorates. The data communication apparatus of the presentinvention utilizes this feature so as to induce an identification errorin the all-possible attacks by the third party using all the thresholds,thereby causing the eavesdropping to be difficult. Particularly, in thecase where the step width of the multi-level signal 15 is set at anorder equal to or smaller than a noise amplitude (spread of a noiseintensity distribution), the data communication apparatus substantiallydisables the multi-level determination by the third party, therebyrealizing an ideal eavesdropping prevention.

As the noise to be overlapped on the signal to be determined (themulti-level signal 15 or the modulated (modulating) signal 14), athermal noise (Gaussian noise) included in a space field or anelectronic device, etc. maybe used, in the case where an electromagneticwave such as a wireless signal is used as the modulated (modulating)signal 14, and a photon number distribution (quantum noise) may be usedin addition to the thermal noise, in the case where the light wave isused. Particularly, signal processing such as recording and replicationis not applicable to a signal using the quantum noise, and thus the stepwidth of the multi-level signal 15 is set by using the quantum noiselevel as a reference, whereby the eavesdropping by the third party isdisabled and an absolute security of the data communication is secured.

As above described, according to the data communication apparatus basedon the first embodiment of the present invention, when the informationdata to be transmitted is encoded as the multi-level signal, theinterval between the signal levels of the multi-level signal 13 is setwith respect to the noise level so as to disable eavesdropping by thethird party. Accordingly, quality of the receiving signal at the time ofthe eavesdropping by the third party is crucially deteriorated, and itis possible to provide a further safe data communication apparatus whichcauses decryption/decoding of the multi-level signal by the third partyto be difficult.

Note that the multi-level encoding section 111 may fluctuate the stepwidth (S1 to S7) of the multi-level signal 13, as shown in FIG. 5,depending on a fluctuation level of each of the levels, that is, thenoise intensity distribution overlapped on each of the levels.Specifically, the interval between the signal levels of the multi-levelsignal 13 is distributed such that respective SN ratios determined basedon respective adjoining two signal levels of the signal to be determinedwhich are inputted to the decision section 212 b become approximatelyuniform. Further, the step width of each of the levels of themulti-level signal 13 is set in a uniform manner, in the case where thenoise level to be overlapped on each of the levels is constant.

Generally, in the case where a light intensity modulated signal whoselight source is a diode laser (LD) is assumed as the modulated signal 14outputted from the modulator section 112, a fluctuation width (the noiselevel) of the modulated (modulating) signal 14 will vary depending onthe levels of the multi-level signal 13 inputted to the diode laser.This results from the fact that the diode laser emits light based on theprinciple of stimulated emission which uses a spontaneous emission lightas a “master light”, and the noise level contained in the modulatedsignal outputted from the diode laser is defined based on a relativeratio of a stimulated emission light level to a spontaneous emissionlight level. That is, the higher an excitation rate of the diode laser(the excitation rate of the diode laser corresponds to a bias current tobe injected) is, the larger a ratio of the stimulated emission lightlevel becomes, and consequently the noise level becomes small. On theother hand, the lower the excitation rate of the diode laser is, thelarger a ratio of the natural emission light level becomes, andconsequently the noise level becomes large. Accordingly, as shown inFIG. 5, the multi-level encoding section 111 sets the step width to belarge in a range where the level of the multi-level signal 13 is small,and sets the step width to be small in a range where the level of themulti-level signal is large, in a non-linear manner, whereby it ispossible to set, in an approximately uniform manner, the respective SNratios of the intervals between the respective adjoining signal levelsof the signal to be determined.

Further, in the case where a light modulated signal is used as themodulated (modulating) signal 14, a SN ratio of a receiving signal willbe determined mainly based on a shot noise as long as a noise caused bythe spontaneous emission light or the thermal noise to be used for anoptical receiver is sufficiently small. Under such condition, the largerthe level of the multi-level signal is, the larger the noise levelincluded in the multi-level signal becomes. Therefore, contrary to thecase of FIG. 5, the multi-level encoding section 111 sets the step widthto be small in the range where the level of the multi-level signal issmall, and sets the step widths to be large in the range where the levelof the multi-level signal is large, whereby it is possible to set, in anapproximately uniformmanner, the respective SN ratios of the intervalsbetween the respective adjoining signal levels of the signal to bedetermined. Accordingly, the quality of the receiving signal at the timeof the eavesdropping by the third party is crucially deteriorated in auniform manner, and it is possible to cause decryption/decoding of themulti-level signal by the third party to be difficult.

Second Embodiment

An overall configuration of a data communication apparatus according toa second embodiment of the present invention is the same as that of thedata communication apparatus as shown in FIG. 1, and thus descriptionthereof will be omitted. The data communication apparatus according tothe second embodiment is different, only with regard to configurationsof a first multi-level code generation section 111 a and a secondmulti-level code generation section 212 a, from the first embodiment.FIG. 6 is a block diagram showing an example of a detailed configurationof the first multi-level code generation section 111 a according to thesecond embodiment of the present invention. In FIG. 6, the firstmulti-level code generation section 111 a has a first random numbersequence generation section 141 and a first multi-level conversionsection 142. The first random number sequence generation section 141includes a pseudo-random number generation section 1411 and aserial/parallel conversion section 1412. Here, an example of a casewhere the number of bits of the multi-level code sequence 12 is 8 bits(m=8) is shown.

The pseudo-random number generation section 1411 generates, based oninputted first key information 11, a binary pseudo-random number series31. The serial/parallel conversion section 1412 performs serial/parallelconversion of the pseudo-random number series 31, and outputs first toeighth modulation pseudo-random number sequences 32 a to 32 h. The firstto eighth modulation pseudo-random number sequences 32 a to 32 h areinputted to the first multi-level conversion section 142. Further, thefirst modulation pseudo-random number sequence 32 a is inputted to themulti-level processing section 111 b. The first multi-level conversionsection 142 converts the first to eighth modulation pseudo-random numbersequences 32 a to 32 h into the multi-level code sequence 12 having2^(m) multi-levels, and then outputs the same to the multi-levelprocessing section 111 b.

FIG. 7 is a block diagram showing an example of a detailed configurationof the second multi-level code generation section 212 a according to thesecond embodiment of the present invention. In FIG. 7, a configurationof the second multi-level code generation section 212 a is basically thesame as that of the first multi-level code generation section 111 a.Note that, in the second multi-level code generation section 212 a,outputs from a serial/parallel conversion section 2412 are referred toas first to eighth demodulation pseudo-random number sequences 42 a to42 h. The second multi-level code generation section 212 a outputs amulti-level code sequence 17 and the first demodulation pseudo-randomnumber sequence 42 a to the decision section 212 b.

FIG. 8 is a diagram illustrating a signal format used for the datatransmitting apparatus according to the second embodiment of the presentinvention. With reference to FIG. 8, a value of the multi-level codesequence 12 used in the present embodiment is determined based on thefirst to eighth modulation pseudo-random number sequences 32 a to 32 h.Further, a level of a multi-level signal is determined based on thevalue of the multi-level code sequence 12 and a value of the informationdata 10. Further, a step width of the multi-level signal is set to beequal to or smaller than a noise level.

The multi-level processing section 111 b allocates respectivelyadjoining levels of the multi-level signal to different values of theinformation data 10 (“0” or “1”) in an alternate manner. For example, inthe levels of the multi-level signal included in an upper half side ofFIG. 8, the multi-level processing section 111 b allocates theinformation data “0” in the case where the multi-level code sequence 12is odd-numbered, and the information data “1” in the case where themulti-level code sequence 12 is even-numbered. Further, in the levels ofthe multi-level signal included in a lower half side of the FIG. 8, themulti-level processing section 111 b allocates the information data “1”in the case where the multi-level code sequence 12 is odd-numbered, andthe information data “0” in the case where multi-level code sequence 12is even-numbered. In other words, a manner of the multi-level processingsection 111 b relating each of the levels of the multi-level signal toeither “0” or “1” is determined based on a value of the first modulationpseudo-random number sequence 32 a which corresponds to a lowest-orderbit of the multi-level code sequence 12. Accordingly, it becomesimpossible for an eavesdropper who does not have key information toidentify data directly, and consequently the eavesdropper is forced totry to identify the key information so as to execute eavesdropping byfirst performing a multi-level determination of all the levels of themulti-level signal.

On the other hand, in the data receiving apparatus, an identificationlevel of a received multi-level signal is determined based on values ofthe first to eighth demodulation pseudo-random number sequences 42 a to42 h. The decision section 212 b decides the value of the informationdata in accordance with a level of the received multi-level signal, theidentification level of the multi-level signal, and a value of the firstdemodulation pseudo-random number sequence 42 a.

Specifically, the decision section 212 b decides the value of theinformation data as “1” in the case where the level of the receivedmulti-level signal is larger than the identification level, and thevalue of the first demodulation pseudo-random number sequence 42 a is“0”, also in the case where the level of the received multi-level signalis smaller than the identification level, and the value of the firstdemodulation pseudo-random number sequence 42 a is “1”. Contrary tothis, the decision section 212 b decides the value of the informationdata as “0” in the case where the level of the received multi-levelsignal is larger than the identification level and the value of thefirst demodulation pseudo-random number sequence 42 a is “1”, and alsoin the case where the level of the received multi-level signal issmaller than the identification level, and the value of the firstdemodulation pseudo-random number sequence 42 a is Note that, theexamples of FIG. 6 and FIG. 7 illustrate cases where the number of themodulation pseudo-random number sequences is 8, however, the number ofthe modulation pseudo-random number sequences is not limited thereto,and can be set arbitrarily.

As above described, according to the present embodiment, in the casewhere the eavesdropper attempts the multi-level determination of themulti-level signal so as to identify the key information, an error inidentification of the multi-level signal will occur, as with a case ofthe first embodiment, since the step-width of the multi-level signal isset to be equal to or smaller than the noise level. Accordingly, thedata communication apparatus according to the second embodiment cancrucially deteriorates quality of a receiving signal at the time ofeavesdropping by a third party, whereby it is possible to provide a safedata communication apparatus which causes decryption/decoding of thereceiving signal to be difficult.

Third Embodiment

In the data communication apparatus according to the second embodiment(see FIG. 6 and FIG. 7), it is necessary to change the first to eighthmodulation pseudo-random number sequences 32 a to 32 h and the value ofthe multi-level code sequence 12 at the same rate as a bit rate of theinformation data 10. Here, a rate of a pseudo-random number series 31(that is, a random number generation rate of a pseudo-random numbergeneration section 1411) is obtained from a product of the bit rate ofthe information data 10 and the number of the bits of the multi-levelcode sequence 12. Therefore, the random number generation rate of thepseudo-random number generation section 1411 increases as the number ofmulti-levels of the multi-level code sequence 12 increases. On the otherhand, a receiving SN ratio of an eavesdropper deteriorates as the numberof the multi-levels increases, and thus the more the number of themulti-levels increases, the more significant identification error theeavesdropper will incur. Accordingly, the more the number of themulti-levels are increased for the sake of security, the more the randomnumber generation rate required to the pseudo-random number generationsection 1411 is increased, which lead to a problem in that it isdifficult to realize such pseudo-random number generation section 1411.The present embodiment aims to solve such problem.

An overall configuration of a data communication apparatus according toa third embodiment of the present invention is the same as that of thedata communication apparatus as shown in FIG. 1, and thus descriptionthereof will be omitted. The data communication apparatus according tothe third embodiment is different, only with regard to configurations ofa first multi-level code generation section 111 a and a secondmulti-level code generation section 212 a, from the second embodiment.Hereinafter, component parts which are the same as those of the secondembodiment are omitted by providing common reference characters, and thedata communication apparatus according to the third embodiment will bedescribed by mainly focusing such components parts that are differentfrom those of the second embodiment.

FIG. 9 is a block diagram showing an example of a detailed configurationof the first multi-level code generation section 111 a according to thethird embodiment of the present invention. In FIG. 9, the firstmulti-level code generation section 111 a has a first random numbersequence generation section 141 and a first multi-level conversionsection 142. The first random number sequence generation section 141includes a pseudo-random number generation section 1411 and aserial/parallel conversion section 1412. Here, an example of a casewhere the number of bits of the multi-level code sequence 12 is 8 bits(m=8) is shown.

In the first multi-level code generation section 111 a, thepseudo-random number generation section 1411 generates, in a similarmanner to the second embodiment (see FIG. 6), a binary pseudo-randomnumber series 31 in accordance with the first key information 11. Theserial/parallel conversion section 1412 performs serial/parallelconversion of the pseudo-random number series 31 and outputs first tofourth modulation pseudo-random number sequences 32 a to 32 d. Here, thenumber of the modulation pseudo-random number sequences outputted fromthe serial/parallel conversion section 1412 is smaller than the numberof bits of a bit sequence to be inputted to the first multi-levelconversion section 142 (that is, an input bit sequence). The first tofourth modulation pseudo-random number sequences 32 a to 32 d areinputted to the first multi-level conversion section 142 as a part ofthe input bit sequence. For example, as shown in FIG. 9, the modulationpseudo-random number sequences 32 a and 32 b, and the modulationpseudo-random number sequences 32 c and 32 d are inputted to low-order 2bits and to high-order 2 bits, respectively, of an 8-bit input bitsequence. Fixed values are inputted to remaining parts of the input bitsequence. The first multi-level conversion section 142 converts theinputted bit sequences into the multi-level code sequence 12 having2^(m)multi-levels and then outputs the same to the multi-levelprocessing section 111 b.

FIG. 10 is a block diagram showing an example of a detailedconfiguration of the second multi-level code generation section 212 aaccording to the third embodiment of the present invention. In FIG. 10,the second multi-level code generation section 212 a has a second randomnumber sequence generation section 241 and a second multi-levelconversion section 242. The second random number sequence generationsection 241 includes a pseudo-random number generation section 2411 anda serial/parallel conversion section 2412.

In the second multi-level code generation section 212 a, thepseudo-random number generation section 2411 generates and outputs,based on the second key information 21, a binary pseudo-random numberseries 41. The serial/parallel conversion section 2412 performsserial/parallel conversion of the pseudo-random number series 41, andoutputs first to fourth demodulation pseudo-random number sequences 42 ato 42 d. Here, the number of the demodulation pseudo-random numbersequences outputted from the serial/parallel conversion section 2412 issmaller than the number of bits of a bit sequence to be inputted to thesecond multi-level conversion section 242 (that is, the input bitsequence). A part of the demodulation pseudo-random number sequencesoutputted from the serial/parallel conversion section 2412 is inputtedto the second multi-level conversion section 242 as a part of the inputbit sequence.

For example, as shown in FIG. 10, the third and the fourth demodulationpseudo-random number sequences 42 c and 42 d are inputted to the secondmulti-level conversion section 242 as high-order bits of the input bitsequence. It is preferable that a position in the input bit sequence tothe second multi-level conversion section 242 to which the demodulationpseudo-random number sequences are to be inputted is the same as that ofa high-order bit in the input bit to the first multi-level conversionsection 142 to which the modulation pseudo-random number sequences areinputted. Fixed values are inputted to remaining bit sequence positionsof the input bit sequence to the second multi-level conversion section242 to which the demodulation pseudo-random number sequences are notinputted. The second multi-level conversion section 242 converts theinput bit sequence into the multi-level code sequence 22 having2^(m)multi-levels and then outputs the same.

FIG. 11 is a diagram illustrating a signal format used for a datatransmitting apparatus according to the third embodiment of the presentinvention. With reference to FIG. 11, in the case where four bits of theinput bit sequence to the first multi-level conversion section 142 arefixed values, the number of the levels which the multi-level codesequence 12 may actually obtain is 16. The level of the multi-levelsignal is determined based the multi-level code sequence 12 and a valueof the information data 10 (“0” or “1”), and thus the number of thelevels which the multi-level signal 13 may obtain is 32. These levelsare divided into 8 groups respectively having four levels respectivelyincluding values which are close to one another. A step width of themulti-level signal in each of the groups is set to be equal to orsmaller than a noise level. Further, it is preferable that a differencebetween a highest-order level and a lowest-order level in each of thegroups is equal to or smaller than the noise level.

Further, the multi-level processing section 111 b allocates, in each ofthe groups, respectively adjoining levels of the multi-level signal todifferent values of the information data 10 (“0” or “1”) in an alternatemanner. For example, in the levels of the multi-level signal included inan upper-half side as shown in FIG. 11, the multi-level processingsection 111 b, allocates the information data “0” in the case where themulti-level code sequence 12 is odd-numbered, and allocates theinformation data “1” in the case where the multi-level code sequence 12is even-numbered. Further, in the levels of the multi-level signalincluded in a lower-half side as shown in FIG. 11, the multi-levelprocessing section 111 b allocates the information data “1” in the casewhere the multi-level code sequence 12 is odd-numbered, and allocatesthe information data “0” in the case where the multi-level code sequence12 is even-numbered. In other words, a manner in which the multi-levelprocessing section 111 b relates each of the levels of the multi-levelsignal to either of “0” or “1” is determined based on a value of thefirst modulation pseudo-random number sequence 32 a which corresponds toa lowest-order bit of the multi-level code sequence 12.

On the other hand, in a data receiving apparatus, an identificationlevel of a received multi-level signal is determined based on values ofthe third and the fourth demodulation pseudo-random number sequences 42c and 42 d. The data receiving apparatus may also use values of thefirst and the second demodulation pseudo-random number sequences 42 aand 42 b when determining the identification level, however, sincefluctuation of the identification level corresponding to the values issmall, an error rate after identification will not deteriorate even ifthe identification level is determined with the fluctuation beingignored. The decision section 212 b decides the value of the informationdata in accordance with the level of the received multi-level signal,the identification level of the multi-level signal, and the value of thefirst demodulation pseudo-random number sequence 42 a.

Specifically, the decision section 212 b decides the value of theinformation data as “1” in the case where the level of the receivedmulti-level signal is greater than the identification level and thevalue of the first demodulation pseudo-random number sequence 42 a is“0”, and also in the case where the level of the received multi-levelsignal is smaller than the identification level and the value of thefirst demodulation pseudo-random number sequence 42 a is “1”. On theother hand, the decision section 212 b decides the value of theinformation data as “0” in the case where the level of the receivedmulti-level signal is greater than the identification level and thevalue of the first demodulation pseudo-random number sequence 42 a is“1”, and also in the case where the level of the received multi-levelsignal is smaller than the identification level and the value of thefirst demodulation pseudo-random number sequence 42 a is “0”.

The random number generation rate required to the pseudo-random numbergeneration section 1411 in the configuration of FIG. 9 is four times ofthe bit rate of the information data 10, since the number of output bits(the number of the modulation pseudo-random number sequences) of theserial/parallel conversion section 1412 is four, and compared to thecase of the configuration of FIG. 6 (8 times of the bit rate of theinformation data 10), the random number generation rate of thepseudo-random number generation section 1411 can be halved.

Note that the fluctuation of the levels of the multi-level signalcorresponding to the first and the second demodulation pseudo-randomnumber sequences 42 a and 42 b which are not used for generating theidentification level leads to a deterioration of a signal level, thatis, an deterioration of an SN ratio, at the time of identification.However, if such deteriorated SN ratio is set so as to satisfy arequired value of the data receiving apparatus 1201, a legitimatereceiving party can identify the multi-level signal without an error.That is, a ratio of a information amplitude to a fluctuation width ofthe multi-level signal corresponding to the low-order bits of thedemodulation pseudo-random number sequences is set so as to satisfy acondition of being greater than the SN ratio acceptable to thelegitimate receiving party. The SN ratio acceptable to the legitimatereceiver is determined based on a bit error rate of data required by thelegitimate receiving party. For example, in optical communications, avalue equal to or smaller than 10⁻¹² are generally used, as anacceptable bit error rate, and for this case, acceptable SN rate isequal to or more than 23 dB.

Further, in the example of FIG. 9, the number of input bits to the firstmulti-level conversion section 142 is 8 bits, and the number of themodulation pseudo-random number sequences is four, and the example showsthat the modulation pseudo-random number sequences are inputted to thehigh-order 2 bits and low-order 2 bits of the input bit sequence to thefirst multi-level conversion section 142, but is merely one example. Thenumber of input bits to the first multi-level conversion section 142 isarbitrary, and the numbers of the modulation pseudo-random numbersequences and the demodulation pseudo-random number sequences can be setarbitrarily in accordance with a ratio of a feasible random numbergeneration rate to a required bit rate. Further, the number of themodulation pseudo-random number sequences to be allocated to thehigh-order bits and low-order bits of the input bit to the firstmulti-level conversion section 142 can be set arbitrarily if itsatisfies a condition where any of the modulation pseudo-random numbersequences is definitely inputted to the lowest-order bit of the inputbit sequence.

As above described, according to the present embodiment, in the casewhere the eavesdropper attempts a multi-level determination of themulti-level signal so as to identify the key information, anidentification error of the multi-level signal occurs in the similarmanner to the first embodiment since the step width of the multi-levelsignal in a single group is set to be equal to or smaller than the noiselevel. Further, the signal levels of the multi-level signal is allocatedappropriately, whereby it is possible to keep, at a low level, anincrease in the random number generation rate required to thepseudo-random number generator, thereby improving the security.Therefore, the data communication apparatus according to the thirdembodiment can crucially deteriorates quality of a receiving signal atthe time of eavesdropping by a third party, whereby it is possible toprovide a safe data communication apparatus which causesdecryption/decoding of the receiving signal to be difficult.

Fourth Embodiment

An overall configuration of a data communication apparatus according toa fourth embodiment of the present invention is the same as that of thedata communication apparatus as shown in FIG. 1, and thus descriptionthereof will be omitted. The data communication apparatus according tothe fourth embodiment is different, only with regard to a configurationof a first multi-level code generation section 111 a, from the thirdembodiment. Hereinafter, component parts which are the same as those ofthe third embodiment are omitted by providing common referencecharacters, and the data communication apparatus according to the fourthembodiment will be described by mainly focusing such components partsthat are different from those of the third embodiment.

FIG. 12 is a block diagram showing an example of a detail configurationof the first multi-level code generation section 111 a according to thefourth embodiment of the present invention. In FIG. 12, the firstmulti-level code generation section 111 a has a first random numbersequence generation section 141, first multi-level conversion section142, and a physical random number generation section 143. The firstrandom number sequence generation section 141 includes a pseudo-randomnumber generation section 1411 and a serial/parallel conversion section1412. Here, an example of a case where the number of bits of themulti-level code sequence 12 is 8 bits (m=8) is shown. A secondmulti-level code generation section 212 a in the present embodiment hasa configuration as shown in FIG. 7, as with the second embodiment.

Next, an action of the data communication apparatus according to thepresent embodiment will be described. Actions of the pseudo-randomnumber generation section 1411 and the serial/parallel conversionsection 1412 are the same as those of the second embodiment. Thephysical random number generation section 143 generates and outputs oneor a plurality of physical random number sequences. In the example ofFIG. 12, the physical random number generation section 143 outputs firstto fourth physical random number sequences 33 a to 33 d. Here, thenumber of modulation pseudo-random number sequences 32 a to 32 doutputted from the serial/parallel conversion section 1412 is set so asto be smaller than the number of bits of the input bit sequence to thefirst multi-level conversion section 142. The first to fourth modulationpseudo-random number sequences 32 a to 32 d are inputted as a part ofthe input bit sequence to the first multi-level conversion section 142.The first to the fourth physical random number sequences 33 a to 33 dare inputted to a remaining part of the input bit sequence. The firstmulti-level conversion section 142 converts the input bit sequence intoa multi-level code sequence 12 having 2^(m) multi-levels and outputs thesame.

FIG. 13 is a diagram illustrating a signal format used for the datatransmitting apparatus according to the fourth embodiment of the presentinvention. The signal format as shown in FIG. 13 corresponds to theconfiguration of the first multi-level code generation section 111 a asshown in FIG. 12. With reference to FIG. 13, the first multi-level codegeneration section 111 a determines high-order 2 bits and low-order 2bits of 8 bits of the multi-level code sequence 12, in accordance withthe modulation pseudo-random number sequences 32 a to 32 d, and alsodetermines intermediate 4 bits in accordance with the physical randomnumber sequences 33 a to 33 d. Therefore, the number of levels of themulti-level code sequence corresponding to the first to fourthmodulation pseudo-random number sequences 32 a to 32 d is 16. A stepwidth of the multi-level signal is set to be equal to or smaller than anoise level. Further, respectively adjoining levels of the multi-levelsignal are allocated to different values of the information data.

On the other hand, in a data receiving apparatus, an identificationlevel of a received multi-level signal is determined, in a similarmanner to the second embodiment, based on values of the third and thefourth demodulation pseudo-random number sequences 42 c and 42 d. In thedecision section 212 b, a value of the information data is decided basedon the level of the multi-level signal, the identification level of themulti-level signal, and the value of the first demodulationpseudo-random number sequence 42 a.

Specifically, the decision section 212 b decides the value of theinformation data as “1” in the case where the level of the receivedmulti-level signal is greater than the identification level and thevalue of the first demodulation pseudo-random number sequence 42 a is“0”, and also in the case where the level of the received multi-levelsignal is smaller than the identification level and the value of thefirst demodulation pseudo-random number sequence 42 a is “1”. On theother hand, the decision section 212 b decides the value of theinformation data as “0” in the case where the level of the receivedmulti-level signal is greater than the identification level and thevalue of the first demodulation pseudo-random number sequence 42 a is“1”, and also in the case where the level of the received multi-levelsignal is smaller than the identification level and the value of thefirst demodulation pseudo-random number sequence 42 a is “0”.

Note that fluctuation of the levels of the multi-level signalcorresponding to the first to the fourth physical random numbersequences which are not used for generating the identification levelleads to a deterioration of a signal level, that is, a deterioration ofan SN ratio, at the time of identification. However, if suchdeteriorated SN ratio is set so as to satisfy a required value of thedata receiving apparatus 1201, a legitimate receiving party can identifythe multi-level signal without error. That is, a ratio of a informationamplitude to a fluctuation width of the multi-level signal correspondingto the physical random number sequence is required to be set so as tosatisfy a condition of being greater than the SN ratio acceptable to thelegitimate receiving party.

As a configuration which can obtain the same effect as the firstmulti-level code generation section 111 as shown in FIG. 12, aconfiguration as shown in FIG. 14A may be considered. FIG. 14A is ablock diagram showing an example of another configuration of the firstmulti-level code generation section 111 a according to the fourthembodiment of the present invention. FIG. 14A is the same, with regardto functional blocks and actions thereof contained in the configuration,as FIG. 12, but is different from FIG. 12 in that FIG. 14A includes abit sequence, as the input bit sequence to the first multi-levelconversion section 142, to which not only the modulation pseudo-randomnumber sequences 32 a to 32 c and the physical random number sequences33 a to 33 b but also fixed values are inputted. FIG. 15 illustrates amulti-level signal format in this exemplary configuration. In this case,fixed values are allocated to 2 bits of the input bit sequence to thefirst multi-level conversion section 142, the number of levels which themulti-level code sequence 12 may obtain is 64. Since the level of themulti-level signal corresponds to the multi-level code sequence 12 andthe value of the information data 10 (“0” or “1”), the number of thelevel to be obtained is 128. These levels are divided into 8 groupsrespectively having 16 levels respectively including values which areclose to one another. The step width of the multi-level signal in eachof the groups is set to be equal to or smaller than the noise level.Further, in each of the groups, respectively adjoining levels of themulti-level signal are allocated to different values of the informationdata. On the other hand, the identification level is determined, in asimilar manner to a case of FIG. 9, based on the values of the third andthe fourth demodulation pseudo-random number sequences 42 c and 42 d.

Further, as a configuration which can obtain the same effect as thefirst multi-level code generation section 111 a as shown in FIG. 12, aconfiguration as shown in FIG. 14A may be considered. FIG. 14B is ablock diagram showing an example of another configuration of the firstmulti-level code generation section 111 a according to the fourthembodiment of the present invention. FIG. 14B is basically the same,with regard to functional blocks and actions thereof contained in theconfiguration, as FIG. 12, but is different from FIG. 12 in that, as apart of the input bit sequence to the first multi-level conversionsection 142, signals generated based on a predetermined rule areinputted instead of the physical random number sequences 33 a to 33 d.In the example as shown in FIG. 14B, signals, which are generated byproviding predetermined delay time to the modulation pseudo-randomnumber sequences 32 a to 32 c, are inputted to the first multi-levelconversion section 142 as the signals generated based on thepredetermined rule.

Note that the examples of FIG. 9 and FIG. 10 shows that the number ofinput bits to the first multi-level conversion section 142 is 8 bit, andthe numbers of the modulation pseudo-random number sequences and thedemodulation pseudo-random number sequences are respectively four, andthe modulation pseudo-random number sequences are inputted to thehigh-order 2 bits and the low-order 2 bits of the input bit sequence tothe first multi-level conversion section 142, but these are merely oneexamples, respectively. The number of the input bits to the firstmulti-level conversion section 142 is arbitrary, and the numbers of themodulation pseudo-random number sequences and the demodulationpseudo-random number sequences can be set arbitrarily in accordance witha ratio of a feasible random number generation rate to a required bitrate. Further, the number of the physical random number sequences can beset arbitrarily if the number of the same is equal to or smaller than adifference between the number of the input bits to the first multi-levelconversion section 142 and the number of the modulation pseudo-randomnumber sequences. Further, selection of whether either of the modulationpseudo-random number sequence or the physical random number sequence, orthe fixed value is to be inputted to respective positions of the inputbit sequence can be set arbitrarily if it satisfies a condition wherethe modulation pseudo-random number sequence is definitely inputted tothe lowest-order bit of the input bit sequence.

As above described, according to the present embodiment, the number ofthe levels which the multi-level signal may obtain is greater than thethird embodiment, and thus the number of the levels of the multi-levelsignal which is likely to be identified erroneously at the time of themulti-level determination by the eavesdropper also increases, wherebyeavesdropping will become difficult. Further, it is possible to keep, ata low level, an increase in the random number generation rate requiredto the pseudo-random number generator, thereby improving the security.Therefore, the data communication apparatus according to the fourthembodiment can crucially deteriorates quality of a receiving signal atthe time of eavesdropping by a third party, whereby it is possible toprovide a safe data communication apparatus which causesdecryption/decoding of the receiving signal to be difficult.

Fifth Embodiment

The fifth embodiment of the present invention aims to keep apseudo-random number generation rate constant and to transmitinformation data 10 at different bit rates. An overall configuration ofa data communication apparatus according to the fifth embodiment of thepresent invention is the same as that of the data communicationapparatus as shown in FIG. 1, and thus description thereof will beomitted. The data communication apparatus according to the fifthembodiment is different, only with regard to configurations of a firstrandom number sequence generation section and a second random numbersequence generation section 241, from the third embodiment. Hereinafter,component parts which are the same as those of the third embodiment areomitted by providing common reference characters, and the datacommunication apparatus according to the third embodiment will bedescribed by mainly focusing such components parts that are differentfrom those of the third embodiment.

FIG. 16 is a block diagram showing an example of a detail configurationof the first random number sequence generation section 141 according tothe fifth embodiment of the present invention. In FIG. 16, the firstrandom number sequence generation section 141 has a pseudo-random numbergeneration section 1411, a first switch 1413, a first serial/parallelconversion section 1414, a second serial/parallel conversion section1415, and a second switch 1416.

Next, an action of the data communication apparatus according to thepresent embodiment will be described. In a similar manner to the secondembodiment, the pseudo-random number generation section 1411 generates abinary pseudo-random number series 31 in accordance with the first keyinformation 11. The first switch 1413 switches, based on a rateselection signal 36 to be inputted, an output destination of thepseudo-random number series 31 between the first serial/parallelconversion section 1414 and the second serial/parallel conversionsection 1415. The first serial/parallel conversion section 1414 performsserial/parallel conversion of the pseudo-random number series 31, andoutputs first to eighth modulation pseudo-random number sequences 34 ato 34 h. The number of the modulation pseudo-random number sequencesoutputted from the first serial/parallel conversion section 1414 is thesame as the number of the input bits to the first multi-level conversionsection 142. The second serial/parallel conversion section 1415 performsserial/parallel conversion of the pseudo-random number series 31 andoutputs a first to a fourth modulation pseudo-random number sequences 35a to 35 d. The number of the modulation pseudo-random number sequencesoutputted from the second serial/parallel conversion section 1415 is setto be smaller than the number of the input bits to the first multi-levelconversion section 142.

The first to eighth modulation pseudo-random number sequences 34 a to 34h outputted from the first serial/parallel conversion section 1414 andthe first to fourth modulation pseudo-random number sequences 35 a to 35d outputted from the second serial/parallel conversion section 1415 areinputted to the second switch 1416. The second switch 1416 selects,based on the rate selection signal 36, either of the inputs from thefirst serial/parallel conversion section 1414 or the secondserial/parallel conversion section 1415, to be outputted to the firstmulti-level conversion section 142. Here, to the second serial/parallelconversion section 1415, the first to the fourth modulationpseudo-random number sequences 35 a to 35 d are inputted, and fixedvalues are also inputted as remaining bit sequences. The configurationand an action of the second random number sequence generation section241 are not shown, but are the same as those of the first random numbersequence generation section 141.

In the case where the first switch 1413 and the second switch 1416 areswitched to the first serial/parallel conversion section 1414 side, thedata communication apparatus according to the present embodimentperforms the same action as that according to the second embodiment. Abit rate of such case is ⅛ of the random number generation rate in thepseudo-random number generation section 1411. On the other hand, thefirst switch 1413 and the second switch 1416 are switched to the secondserial/parallel conversion section 1415 side, the data communicationapparatus according to the present embodiment performs the same actionas that according to the third embodiment. The bit rate of such case is¼ of the random number generation rate in the pseudo-random numbergeneration section 1411. In this manner, a plurality of serial/parallelconversion sections, which respectively output different numbers ofmodulation pseudo-random number sequences, is prepared and used byswitching therebetween, whereby it is possible to correspond todifferent bit rates in spite of being a single pseudo-random numbergeneration rate. That is, since a product of the number of themodulation pseudo-random number sequence and the bit rate is equal tothe pseudo-random number generation rate, and thus it is possible tovary the bit rate by switching the number of the modulationpseudo-random number sequences, which is limited to a case whereremaining configuration blocks which are not shown in FIG. 16 can beadapted to any transmittable bit rates.

An exemplary configuration of FIG. 16 is merely an example, and anyconfiguration may be possible if the bit rate can be switched byswitching the number of the modulation pseudo-random number sequenceswhile the pseudo-random number generation rate is kept constant.Further, the value of the bit rate to be switched is not limited to two,and can be set arbitrarily as necessary.

As above described, according to the present embodiment, it is possibleto respond to a plurality of bit rates while the random numbergeneration rate of the pseudo-random number generation section is keptconstant.

Note that each of the data communication apparatuses according to thefirst to the fifth embodiments may have a configuration which combinesfeatures of the remaining embodiments. Further, processing performed byeach of the data transmitting apparatuses, the data receivingapparatuses, and the data communication apparatuses according to theabove-described first to fifth embodiments may be respectively regardedas a data transmitting method, a data receiving method, and a datacommunication method, each of which cause a series of processingprocedure to be executed.

Further, the above-described data transmitting method, the datareceiving method, and the data communication method may be realized bycausing a CPU to interpret and execute predetermined program data whichis capable of executing the above-described processing procedure storedin a storage device (such as a ROM, a RAM, and a hard disk). In suchcase, the program data may be executed after being stored in the storagedevice via a storage medium, or may be executed directly from thestorage medium. Note that the storage medium includes a ROM, a RAM, asemiconductor memory such as a flash memory, a magnetic disk memory suchas a flexible disk and a hard disk, an optical disk such as a CD-ROM, aDVD, and a BD, a memory card, or the like. Further, the storage mediumis a notion including a communication medium such as a telephone lineand a carrier line.

The data communication apparatus according to the present invention isuseful as a safe secret communication apparatus which is unsusceptibleto eavesdropping/interception.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

1. A data transmitting apparatus for encrypting information data byusing predetermined key information and for performing secretcommunication with a receiving apparatus, the data transmittingapparatus comprising: a multi-level code generation section forgenerating, based on the predetermined key information, a multi-levelcode sequence in which a signal level changes so as to approximatelyrepresent random numbers; a multi-level processing section for combiningthe multi-level code sequence and the information data, and generating amulti-level signal having a plurality of levels corresponding to thecombination of the multi-level code sequence and the information data;and a modulator section for performing predetermined modulationprocessing on the multi-level signal, and outputting a modulated signal,wherein the multi-level code generation section includes: a randomnumber sequence generation section for generating, based on thepredetermined key information, a plurality of modulation pseudo-randomnumber sequences; and a multi-level conversion, into which a pluralityof bit sequences including at least part of the plurality of modulationpseudo-random number sequences is inputted as an input bit sequence, forconverting the input bit sequence into the multi-level code sequence,wherein the input bit sequence inputted into the multi-level conversionsection is greater in a number of digits than each of the plurality ofmodulation pseudo-random number sequences generated by the random numbersequence generation section, and wherein the multi-level processingsection allocates different values of the information data to adjoiningmulti-levels of the multi-level signal.
 2. The data transmittingapparatus according to claim 1, wherein at least one of the plurality ofmodulation pseudo-random number sequences is inputted to the multi-levelconversion section as a lowest-order bit of the input bit sequence. 3.The data transmitting apparatus according to claim 1, wherein themulti-level code generation section further includes a physical randomnumber generation section for generating one or more physical randomnumber sequences, and wherein the one or more physical random numbersequences are inputted, into the multi-level conversion section, asremaining bit sequences of the input bit sequence after excluding the atleast part of the plurality of modulation pseudo-random numbersequences.
 4. The data transmitting apparatus according to claim 1,wherein fixed values are inputted, into the multi-level conversionsection, as remaining bit sequences of the input bit sequence afterexcluding the at least part of the plurality of modulation pseudo-randomnumber sequences.
 5. The data transmitting apparatus according to claim1, wherein the multi-level code generation section further includes aphysical random number generation section for generating one or morephysical random number sequences, and wherein the one or more physicalrandom number sequences are inputted, into the multi-level conversionsection, as a part of the plurality of bit sequences of the input bitsequence after excluding the at least part of the plurality ofmodulation pseudo-random number sequences, and wherein fixed values areinputted, into the multi-level conversion section, as remaining bitsequences of the input bit sequence.
 6. The data transmitting apparatusaccording to claim 1, wherein a signal generated based on apredetermined rule is inputted, into the multi-level conversion section,as remaining bit sequences of the input bit sequence after excluding theat least part of the plurality of modulation pseudo-random numbersequences.
 7. The data transmitting apparatus according to claim 6,wherein the signal generated based on the predetermined rule isgenerated by delaying a part or a whole of the plurality of modulationpseudo-random number sequences by a predetermined time period.
 8. Thedata transmitting apparatus according to claim 3, wherein a ratio of aninformation amplitude, which corresponds to an amplitude of theinformation data, to a fluctuation width of the multi-level signal isgreater than a signal-to-noise ratio that is acceptable to a legitimatereceiving party.
 9. The data transmitting apparatus according to claim1, wherein the random number sequence generation section includes: apseudo-random number generation section for generating, based on thepredetermined key information, a pseudo-random number series which is ina binary format; and a serial/parallel conversion section for performingserial/parallel conversion of the pseudo-random number series generatedby the pseudo-random number generation section, and outputting theplurality of modulation pseudo-random number sequences.
 10. The datatransmitting apparatus according to claim 1, wherein the random numbersequence generation section includes: a pseudo-random number generationsection for generating, based on the predetermined key information, apseudo-random number series which is in a binary format; a plurality ofserial/parallel conversion sections for performing serial/parallelconversion of the pseudo-random number series generated by thepseudo-random number generation section and outputting the plurality ofmodulation pseudo-random number sequences; a first switch for switching,based on a rate selection signal, an output destination of thepseudo-random number series generated by the pseudo-random numbergeneration section, between the plurality of serial/parallel conversionsections; and a second switch for selecting, based on the rate selectionsignal, and outputting the plurality of modulation pseudo-random numbersequences outputted from the plurality of serial/parallel conversionsections, and wherein the plurality of serial/parallel conversionsections output respectively different numbers of the plurality ofmodulation pseudo-random number sequences.
 11. A data receivingapparatus for receiving information data encrypted by usingpredetermined key information and for performing secret communicationwith a transmitting apparatus, the data receiving apparatus comprising:a multi-level code generation section for generating, based on thepredetermined key information, a multi-level code sequence in which asignal level changes so as to approximately represent random numbers; ademodulator section for demodulating, in a predetermined demodulationmethod, a modulated signal received from the transmitting apparatus, andoutputting, as a result of the demodulating, a multi-level signal havinga plurality of levels corresponding to a combination of the informationdata and the multi-level code sequence; and a decision section fordeciding, based on the multi-level code sequence, different values ofthe information data that are allocated to adjoining multi-levels offrom the multi-level signal, wherein the multi-level code generationsection includes: a random number sequence generation section forgenerating, based on the predetermined key information, a plurality ofdemodulation pseudo-random number sequences; and a multi-levelconversion section, into which a plurality of bit sequences including atleast part of the plurality of demodulation pseudo-random numbersequences is inputted as an input bit sequence, for converting and whichconverts the input bit sequence into the multi-level code sequence, andwherein the input bit sequence inputted into the multi-level conversionsection is greater in a number of digits than each of the plurality ofdemodulation pseudo-random number sequences generated by the randomnumber sequence generation section.
 12. The data receiving apparatusaccording to claim 11, wherein fixed values are inputted, into themulti-level conversion section, as remaining bit sequences of the inputbit sequence after excluding the at least part of the plurality ofdemodulation pseudo-random number sequences.
 13. The data receivingapparatus according to claim 11, wherein a signal generated based on apredetermined rule is inputted, into the multi-level conversion section,as remaining bit sequences of the input bit sequence after excluding theat least part of the plurality of demodulation pseudo-random numbersequences.
 14. The data receiving apparatus according to claim 11,wherein the signal generated based on the predetermined rule isgenerated by delaying a part or a whole of the plurality of demodulationpseudo-random number sequences by a predetermined time period.
 15. Thedata receiving apparatus according to claim 11, wherein a ratio of aninformation amplitude, which corresponds to an amplitude of theinformation data to a fluctuation width of the multi-level signal, whichcorresponds to remaining bit sequences, after excluding the plurality ofdemodulation pseudo-random number sequences, of the input bit sequence,is greater than a signal-to-noise ratio that is acceptable to alegitimate receiving party.
 16. The data receiving apparatus accordingto claim 11, wherein the random number sequence generation sectionincludes: a pseudo-random number generation section for generating,based on the predetermined key information, a pseudo-random numberseries which is in a binary format; and a serial/parallel conversionsection for performing serial/parallel conversion of the pseudo-randomnumber series generated by the pseudo-random number generation section,and outputting the plurality of demodulation pseudo-random numbersequences.
 17. The data receiving according to claim 11, wherein therandom number sequence generation section includes: a pseudo-randomnumber generation section for generating, based on the predetermined keyinformation, a pseudo-random number series which is in a binary format;a plurality of serial/parallel conversion sections for performingserial/parallel conversion of the pseudo-random number series generatedby the pseudo-random number generation section and outputting theplurality of demodulation pseudo-random number sequences; a first switchfor switching, based on a rate selection signal, an output destinationof the pseudo-random number series generated by the pseudo-random numbergeneration section, between the plurality of the serial/parallelconversion sections; and a second switch for selecting, based on therate selection signal, and outputting the plurality of demodulationpseudo-random number sequences series outputted from the plurality ofserial/parallel conversion sections, and wherein the plurality ofserial/parallel conversion sections outputs respectively differentnumbers of the plurality of demodulation pseudo-random number sequences.18. A data transmitting method for encrypting information data by usingpredetermined key information and for performing secret communicationwith a receiving apparatus, the data transmitting method comprising: amulti-level code generation step of generating, based on thepredetermined key information, a multi-level code sequence in which asignal level changes so as to approximately represent random numbers; amulti-level processing step of combining the multi-level code sequenceand the information data, and generating a multi-level signal having aplurality of levels corresponding to the combination of the multi-levelcode sequence and the information data; and a modulation step ofperforming predetermined modulation processing on the multi-levelsignal, and outputting a modulated signal, wherein the multi-level codegeneration step includes: a random number sequence generation step ofgenerating, based on the predetermined key information, a plurality ofmodulation pseudo-random number sequences; and a multi-level conversionstep of inputting, as an input bit sequence, a plurality of bitsequences including at least part of the plurality of modulationpseudo-random number sequences, and converting the input bit sequenceinto the multi-level code sequence, wherein the input bit sequence isgreater in a number of digits than each of the plurality of modulationpseudo-random number sequences, and wherein the multi-level processingstep allocates different values of the information data to adjoiningmulti-levels of the multi-level signal.
 19. A data receiving method forreceiving information data encrypted by using predetermined keyinformation and for performing secret communication with a transmittingapparatus, the data receiving method comprising: a multi-level codegeneration step of generating, based on the predetermined keyinformation, a multi-level code sequence in which a signal level changesso as to approximately represent random numbers; a demodulation step ofdemodulating, in a predetermined demodulation method, a modulated signalreceived from the transmitting apparatus, and outputting, as a result ofthe demodulating, a multi-level signal having a plurality of levelscorresponding to a combination of the information data and themulti-level code sequence; and a decision step of deciding, based on themulti-level code sequence, different values of the information data thatare allocated to adjoining multi-levels of from the multi-level signal,wherein the multi-level code generation step includes: a random numbersequence generation step of generating, based on the predetermined keyinformation, a plurality of demodulation pseudo-random number sequences;and a multi-level conversion step of inputting, as an input bitsequence, a plurality of bit sequences including at least part of theplurality of demodulation pseudo-random number sequences, and convertingthe input bit sequence into the multi-level code sequence, and whereinthe input bit sequence is greater in a number of digits than each of theplurality of demodulation pseudo-random number sequences.